Apparatus for separating dc current and ac current components of a composite signal

ABSTRACT

An apparatus for separating dc current and ac current signal portions of a composite signal. The apparatus has a main current path for carrying the composite signal. This path splits into a dc current path and into two dc blocked ac current paths. At the node a virtual ground potential for the ac current signal portions is generated with the aid of an inverting amplifier with negative feedback located in one of the ac current paths. Thus, only the dc current portions flow over the dc current path. In a preferred embodiment, the dc blocking is implemented by a capacitor in each ac current path, and the inverting amplifier is an operational amplifier, whose non-inverting input is at ground potential, whose inverting input is connected with the respective capacitor and whose output is connected both with the inverting input via an ohmic negative feedback resistor and with an impedance in the other ac current path.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 061,379 filed June 15,1987, now U.S. Pat. No. 4,896,351, which is a continuation-in-part ofcommonly-owned pending application Ser. No. 872,063, filed June 9, 1986now abandoned. The entire disclosure of this patent application(including the drawings) is hereby incorporated herein as if fully setforth.

BACKGROUND OF THE INVENTION

The invention relates to an apparatus for separating dc current and accurrent components of a composite signal. It relates in particular to acircuit arrangement for separating dc and ac current components of acomposite signal in a subscriber line station.

The necessity of separating signals arises in general in measurementcircuits.

In this connection, a special application example outside this rangeshould be mentioned, namely the conditions of modern subscriber linecircuits for connecting subscriber lines to digital time divisionmultiplexing telephone systems.

For handling a composite signal which occurs in subscriber lines in theform of a loop current superposed by voice signals, subscriber linecircuits usually have to derive loop status indications from the dccurrent portion of the loop current and to simulate the correct accurrent impedance for an optimal signal matching of the voice signal tobe transmitted.

In the interest of an optimal integration capacity, modern subscriberline circuits are subdivided into different hardware modules, of which ahigh voltage power module and a low voltage module are configured asintegrated circuits, while a safety element module and a test module areassembled with discrete components.

While such tasks as call recognition, voice transmission, theestablishing of line symmetry, subscriber line drive and the separationof longitudinal current components are performed by the high voltagepower module, the low voltage module is used for feeding charge pulses,for realizing different monitoring functions, for setting the two wireimpedance, and for detecting the loop status.

When using this type of configuration a sum signal forwarded by thepower module and comprising dc and ac components must be separated intoits constituents to obtain control variables with which the requestedsupply characteristics in form of a respective dc current resistance anda requested impedance can be set.

In principle, this kind of signal separation can be obtained with theuse of RC combinations. However, in the frequency range of the voiceband (300 Hz to 3400 Hz), which is of interest here, capacitance valuesof approximately 100 μF are required. Moreover, the dc current signalsoccur with dc components of either polarity, so that nonpolar capacitorsmust be used. These requirements are met only by foil capacitors, whichare both bulky and expensive.

It is therefore an object of the invention to provide an apparatus whichseparates as completely as possible the dc and ac current components ofa composite signal.

It is another object of the invention to provide an apparatus forseparating the dc and ac current components of a composite signal in acompact and cost-effective manner.

Yet another object of the invention is to provide an apparatus forseparating the dc and ac current components of a composite signal in thevoice band.

A further object of the invention is to provide an apparatus whichseparates the dc and ac current components of a composite signal suchthat even ac current components of high amplitudes are not distorted.

Yet a further object is, in general, to improve on known apparatus ofthis type.

SUMMARY OF THE INVENTION

Broadly, the invention is directed to an apparatus for separating dccurrent and ac current components of a composite signal. The apparatuscomprises a main current path having a node where it branches out into afirst and second circuit. The former has a resistor for receiving the dccurrent component. The later includes an impedance for tapping the accurrent component, dc blocking means and self-compensating means. The dcblocking means prevent dc current components from reaching theimpedance, and the self-compensating means offset voltage drops in otherparts of the ac circuit such that--at least in a frequency range ofinterest--the node is kept at ground potential for the ac currentcomponents.

According to one embodiment of the invention the first circuit includesa first current path having the resistor path, and the second circuitincludes a second and a third current path. The dc blocking meanscomprise a dc blocking filter in each current path, for preventing dccurrent components from entering the path. The third pathincludes--behind its dc blocking filter--an inverting amplifier. Theoutput signal of this amplifier is superposed on the voltage drop in thesecond path and is set in such a manner that it compensates for thevoltage drop in at least the frequency range of interest, so that inthis range the node acts as a virtual ground for the ac currentcomponents.

In accordance with a more specific aspect of invention, the dc blockingfilters are capacitors and the inverting amplifier is an operationalamplifier, whose non-inverting input is tied to ground potential, whoseinverting input is connected with the respective capacitor and whoseoutput is connected with the inverting input via an ohmic negativefeedback resistor and, in addition, with the impedance of the secondpath. The capacitors which are implemented in this case may have aconsiderably reduced capacitance, e.g. less than 10 μf for compositesignals having ac components in the voice band.

If the inverting amplifier is an operational amplifier having an ohmicnegative feedback resistor, this resistor is preferably shunted by twooppositively poled diodes. These diodes have threshold values which willlimit ac voltage constituents having amplitudes which would overdrivethe operational amplifier.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following detailed description ofpreferred embodiments of the invention, as illustrated in theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic circuit diagram of a first embodiment of thisinvention.

FIG. 2 shows a schematic circuit diagram of a second embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, it is assumed that the signal source which supplies acomposite signal I_(DC) +I_(AC) consisting of ac and dc currentportions, is a constant current source which lies in a main current path4. This current path branches at a node VE into a first, second andthird current path 1, 2, and 3, respectively. Current path 1 includes anohmic resistor R_(DC), where the dc voltage portions of the compositesignal are to be tapped. Current path 2 includes a capacitor C as wellas an impedance Z_(AC), where the ac voltage portions of the compositesignal are to be tapped, for example via an operational amplifier (notshown).

The third current path 3 also includes a capacitor C, which can be ofthe same capacitance as the capacitor located in the second current path2, as well as an operational amplifier OP whose non-inverting input (+)is tied to ground potential and whose inverting input (-) is connectedwith the connection of capacitor C which faces away from the node VE.The output of the operational amplifier OP is connected with theinverting input via a negative feedback resistor Rr.

In addition, the output of the operational amplifier OP is alsoconnected with the connection of impedance Z_(AC) which faces away fromcapacitor C of the second current path 2.

By setting the gain of the operational amplifier OP accordingly, thevoltage drop across the switching elements of the second current path 2,i.e. capacitor C and impedance Z_(AC), is compensated for by the voltageat the output of operational amplifier OP. Since only ac currents I_(AC)can flow across the second and the third current path 2 and 3,respectively, a virtual ground potential is created for ac currentsignals at node VE. This means that, as desired, only the dc currentcomponents I_(DC) flow via resistor R_(DC) through the first currentpath 1.

With a circuit arrangement for the voice band between 300 Hz and 3440Hz, which acts like a band pass filter with a pass band that lies,however, far below the voice band, capacitors C in both current paths 2and 3 have only a capacitance of 1 μF each, and the attenuation of theac current portions in the dc current path is larger than 60 dB.

Typically, the circuit of FIG. 1 is used in electronic subscriberinterconnect circuits for the connection of subscriber lines to digitaltime multiplex telephone switching systems. In such an application, acsignals lying outside of the voice band also appear on the current pathconducting the combined signals. These signals, which may be strongerthan the signals in the voice band by an order of magnitude, can be ofgreat enough amplitude to drive the operational amplifier beyond itsdynamic range and are then distorted. The most significant of these acsignals are the dial pulses which are produced by the periodic openingand closing of the battery current circuit, and thus appear withamplitudes as high as 60 volts.

To avoid such a signal distortion, oppositively poled diodes D1 and D2are placed in parallel with the feedback resistance Rr (FIG. 2). Thethresholds of diodes D1 and D2 are so selected that ac voltagesconstituents with amplitudes that would lead to overdriving theoperational amplifier OP, thus especially the dial pulses already noted,are limited.

Having thus described the invention with particular reference to thepreferred form thereof, it will be obvious to those skilled in the artto which the invention pertains, after understanding the invention, thatvarious changes and modifications may be made therein without departingfrom the spirit and scope of the invention as defined by the claimsappended hereto.

We claim:
 1. An apparatus for separating dc current and ac currentcomponents of a composite signal, comprising:(a) a main current path forcarrying the composite signal, said current path having a node where itbranches out into: (b) a first current path including a resistor forreceiving the dc current components; (c) a second current path includingan impedance for tapping the ac current components and a first dcblocking filter for preventing the dc current components from flowingthrough the impedance, said first dc blocking filter being disposedbetween the node and the impedance; and (d) a third current pathincluding an inverting amplifier and a second dc blocking filter forpreventing dc current components from flowing through the invertingamplifier, said second dc blocking filter being disposed between thenode and the inverting amplifier, wherein: the impedance is disposedbetween the first dc blocking filter and the inverting amplifier and theoutput signal of the inverting amplifier is superimposed on the voltagedrop in the second current path and is set is such a manner that itcompensates for said voltage drop in at least a specific frequency rangeof interest so that the node acts as a virtual ground for the ac currentcomponents in said frequency range; each dc blocking filter comprises acapacitor; and the inverting amplifier comprises an operationalamplifier, whose non-inverting input is tied to ground potential, whoseinverting input is connected to the respective capacitor and whoseoutput is connected both with the inverting input via an ohmic negativefeedback resistor and with the impedance.
 2. An apparatus according toclaim 1, wherein the specific frequency range of interest lies between300 Hz and 3440 Hz.
 3. An apparatus according to claim 1, wherein bothcapacitors have approximately the same capacitance.
 4. An apparatusaccording to claim 2, wherein both capacitors have a capacitance of lessthan 10 μf.